[backend/federation] Make sure DateTime objects are being converted to Utc when (de)serializing activities

Iceshrimp.Backend/Core/Extensions/MvcBuilderExtensions.cs

@@ -40,6 +40,16 @@ public static class MvcBuilderExtensions
 		return builder;
 	}
 
+	public static IMvcBuilder ConfigureNewtonsoftJson(this IMvcBuilder builder)
+	{
+		Newtonsoft.Json.JsonConvert.DefaultSettings = () => new Newtonsoft.Json.JsonSerializerSettings
+		{
+			DateTimeZoneHandling = Newtonsoft.Json.DateTimeZoneHandling.Utc
+		};
+
+		return builder;
+	}
+
 	public static IMvcBuilder AddModelBindingProviders(this IMvcBuilder builder)
 	{
 		builder.Services.AddOptions<MvcOptions>()

Iceshrimp.Backend/Core/Federation/ActivityStreams/LdHelpers.cs

@@ -56,12 +56,12 @@ public static class LdHelpers
 
 	public static readonly JsonSerializerSettings JsonSerializerSettings = new()
 	{
-		NullValueHandling = NullValueHandling.Ignore, DateTimeZoneHandling = DateTimeZoneHandling.Local
+		NullValueHandling = NullValueHandling.Ignore, DateTimeZoneHandling = DateTimeZoneHandling.Utc
 	};
 
 	private static readonly JsonSerializer JsonSerializer = new()
 	{
-		NullValueHandling = NullValueHandling.Ignore, DateTimeZoneHandling = DateTimeZoneHandling.Local
+		NullValueHandling = NullValueHandling.Ignore, DateTimeZoneHandling = DateTimeZoneHandling.Utc
 	};
 
 	private static IEnumerable<string> ASForceArray => ["tag", "attachment", "to", "cc", "bcc", "bto"];

Iceshrimp.Backend/Startup.cs

@@ -12,6 +12,7 @@ builder.Configuration.AddCustomConfiguration();
 
 builder.Services.AddControllers()
        .AddNewtonsoftJson() //TODO: remove once dotNetRdf switches to System.Text.Json (or we switch to LinkedData.NET)
+       .ConfigureNewtonsoftJson()
        .AddMultiFormatter()
        .AddModelBindingProviders()
        .AddValueProviderFactories()